1. Field of the Invention
The invention relates to a vehicle control system and a vehicle control method. In particular, the invention relates to a vehicle control system and a vehicle control method for a vehicle that employs an electric oil pump to cool a rotating electrical machine.
2. Description of Related Art
In a vehicle that is mounted with a rotating electrical machine, an oil pump for circulating a coolant that cools the rotating electrical machine is employed. In a vehicle that is mounted with an engine as well as a rotating electrical machine, a mechanism oil pump that is connected to an output rotary shaft of the engine can be employed. When the engine stops, this mechanical oil pump also stops operating. Thus, an electric oil pump that is driven regardless of the operation of the engine is employed, so that the rotating electrical machine can be cooled while the engine is stopped.
For example, it is described in Japanese Patent Application Publication No. 2004-256063 (JP-2004-256063 A) that, in a control apparatus for a vehicle, when the discharge rate of a mechanical oil pump that is driven by an engine during the cut-off of fuel supply becomes insufficient, an oil pump driven by a motor-generator or an electric oil pump is driven.
Incidentally, as an art related to the invention, it is described in Japanese Patent Application Publication No. 2007-112290 (JP-2007-112290 A) that the optimal rotational speed is set in such a manner as to increase as the carrier frequency used for a drive circuit of an electric motor increases when a vehicle is equipped with an internal combustion engine and the electric motor as motive power output units.
As described in Japanese Patent Application Publication No. 2007-112290 (JP-2007-112290 A), the carrier frequency in the drive circuit of a rotating electrical machine may be changed in accordance with the traveling state of the vehicle. For example, when the rotational speed of the rotating electrical machine is high while the vehicle travels at high speed, the carrier frequency is set high in order to maintain the controllability thereof. Besides, on the contrary, when the rotational speed of the rotating electrical machine is low while the vehicle travels at low speed, the carrier frequency is set low. When the carrier frequency becomes low, the amplitude of ripple noise on a drive signal of the rotating electrical machine becomes large. Then, the loss in the rotating electrical machine increases, and the temperature of the rotating electrical machine rises.
In order to cool the rotating electrical machine, it is possible to employ a mechanical oil pump or an electric oil pump. However, since the mechanical oil pump is driven by an engine, the discharge rate of a coolant is determined in accordance with the rotational speed of the engine. In this manner, the cooling capacity of the mechanical oil pump changes depending on the operation state of the engine. Accordingly, if the mechanical oil pump is employed to cool the rotating electrical machine when the temperature of the rotating electrical machine has risen due to a change in the carrier frequency, the rotating electrical machine is insufficiently cooled in some cases.